1. Field of the Invention
The present invention relates to a program input/output control device, and more particularly to a control device for outputting a selected one of a plurality of machine control programs to a workpiece processing machine such as a numerically controlled machine, in response to program selection signals received from an external device.
2. Discussion of the Prior Art
A known numerically controlled machine (NC machine) is operated according to a selected one of machine control programs such as machining or other processing programs, which are stored in a memory such that the programs are selectable with respective program numbers. The selected machine control program is outputted to the NC machine for performing a corresponding operation to machine or otherwise process a workpiece. The desired machine control program is selected by entering the corresponding program number through an operator's control panel of the NC machine.
In an unmanned or unattended, fully automatic manufacturing line, however, the machine control programs (NC programs) for each NC machine are not designated by the machine operator through the operator's control panel of the NC machine, but are automatically selected in response to program selection signals received from an external device, so-called a sequence controller, which is adapted to identify different types of workpieces supplied to the machine, by using a suitable sensor. The program selection signals are sent to a numerical control device of the NC machine, so that the workpieces are machined or otherwise processed according to the corresponding machine control programs.
Conventionally, the program numbers representing the respective machine control programs are directly designated by corresponding combinations of the low/high levels of program selection signals sent from an external device to a plurality of program selection terminals of the numerical control device of the NC machine. In this direct designation system, the required number of the signal lines connected to the program selection terminals tends to be considerably large, particularly where the number of the machine control programs available is relatively large. If the operation programs are identified by program numbers 1 through n, the required number of combinations of the low/high levels of the program selection signals is equal to "n", and therefore the required number of the signal lines (program selection terminals) is equal to log.sub.2 n. Where n=9999, for example, a total of 14 program selection terminals (signal lines) are required for permitting the program selection signals to identify the machine control programs. Consequently, the cost of the overall system including the program selection terminals, signal line cables and connectors tends to be increased.
Although the number of the machine control programs available for use is equal to "n", the number of the machine control programs which are actually used or expected to be used during a given period for processing the workpieces on the relevant NC machine is usually considerably smaller than "n". In this sense, the signal lines and the corresponding program selection terminals used in the conventional system are not effectively utilized.